Purge-and-trap combined with high-resolution gas chromatography and detection by mass spectrometry was evaluated for the analysis of 27 volatile organic compounds (VOCs) in marine water samples down to ng l-1 concentration levels. The target compounds included chlorinated alkanes and alkenes, monocyclic aromatic hydrocarbons and chlorinated monocyclic aromatic hydrocarbons and covered a wide range of VOCs of environmental interest. Limits of detection ranged from 0.15 ng l-1 to 6.57 ng l-1 for all VOCs, except for dichloromethane (41.07 ng l-1), chloroform (19.74 ng l-1), benzene (22.05 ng l-1) and 1,4-dichlorobenzene (20.43 ng l-1), Precision and accuracy were determined at a concentration level of 25.97 to 66.68 ng l-1. Besides method validation, emphasis was put on quality control and assessment during routine determination of VOCs in marine water samples. Analytical quality control charts were plotted for all VOCs and a standard addition test was performed, as proposed by the QUASIMEME (Quality Assurance of Information in Marine Environmental Monitoring Programmes in Europe) working group. The analytical charts were incorporated in a working scheme containing guidelines to be applied during routine determinations, ensuring the long time reliability of the analytical method. Results yielded by the QUASIMEME interlaboratory exercise on organohalogen measurements in seawater are presented. The exercise was attended by seven out of eight laboratories who agreed to participate. Samples taken along the Scheldt estuary, from Breskens (The Netherlands) to Temse (Antwerp, Belgium) were analysed according to the developed technique. Concentrations as low as 0.33 ng l-1 (1,2-dichloropropane) were detected near the mouth of the river Scheldt, while concentrations up to 326 ng l-1 for tetrachloroethene and 461 ng l-1 for cyclohexane were found in the vicinity of Antwerp.